Retractable underbody truck ramp and method of making

ABSTRACT

A retractable ramp is telescopable to a stowed position within a rearwardly open, longitudinally extending storage chamber within a cargo vehicle frame below a cargo carrying floor therein. It can be pulled from the vehicle, tilted downwardly so that its rear end engages the ground, and then its front end can be raised and connected to the vehicle adjacent the floor of the cargo compartment to serve as an unloading or loading platform for the vehicle floor. The ramp includes carriage arms pivotally connected to the ramp assembly near its front end and extending forwardly into the storage chamber to connect to glides which are slideable along the chamber. A force asserting, weight counterbalancing mechanism is pivotally connected between the carriage arm assembly and the ramp, and positioned to exert power to assist manual movement of the front end of the ramp from an initial position in which it is pulled from the storage chamber to a raised, operatively deployed position attached adjacent the ramp floor. The ramp is so positioned in the stowing chamber as it is being stowed, and as it is being withdrawn, that it virtually floats with respect to the walls of the storage chamber and does not generate appreciable frictional forces which add to the physical exertion required to stow and remove the ramp.

This is a divisional of copending application(s) Ser. No. 07/681,977filed on Apr. 8, 1991 now U.S. Pat. No. 5,199,150.

BACKGROUND OF THE INVENTION

The present invention relates particularly to truck-carried ramps whichstow within the frame of the vehicle below the cargo carrying body ofthe motorized truck, and are adaptable to be removed rearwardly to aposition in which the ramp assumes a tilted position resting on theground and connected to the vehicle adjacent the floor of the cargocompartment so as to be useable for cargo loading and unloadingpurposes. Typically, support arms have been pivotally attached beneaththe front ends of such ramps and and the front ends of the arms havebeen pivotally connected to the vehicle frame within the ramp storagechamber. These support arms have supported the front end of the rampwhen it is initially extended from the vehicle until such time as thefront end could be raised vertically and pulled rearwardly to hook intoopenings provided in the vehicle frame which retained the front end ofthe ramp in a raised position in which the floor of the front end of theramp was substantially at the level of the floor of the cargocompartment of the truck. In this operatively deployed position of theramp, hand trucks carrying considerable loads easily transfer betweenthe ramp and the floor of the cargo container.

Applicant's assignee has marketed retractable underbody truck ramps ofthis type for a number of years and the present invention is directed tocertain new and useful improvements which have been made in suchtruck-ramp structures. Applicant's assignee has manufactured a range oframps of differing length and weight for use with cargo-carrying trucksof varying size and floor level. For example, applicant's assignee hasmanufactured and marketed ramps ranging from 10 to 16 feet in length,and weighing between 80 and 140 pounds, in conjunction with appropriatetrack assemblies forming ramp storage chambers which are adapted to bewelded to the truck body frame in a position to receive both the stowedramp floors and the front end support arms which are folded up to theramp floors to also be received in the longitudinally extending rampstorage chambers. To provide attachment devices at a proper level tosecure the front end of the ramp when it is removed from the storagechamber and raised, notches have been cut in the rear apron of the truckbody at cargo floor level to receive hooks provided on the front end ofthe ramp. Rollers have been mounted at the front and rear ends of theramp in a position to support the ramp above storage frame membersforming tracks on which the rollers move, as the ramps and arms arepushed telescopically into the ramp stowing chamber beneath the truckbody floor. Because cargo truck specifications vary with themanufacturer of the cargo truck, applicant's assignee's ramps had to bedesigned to accommodate to trucks having variable vertical distancesbetween the storage chambers and beds of the truck, and to truck bodieshaving aprons which projected different distances.

With ramps of this prior art construction, a truck driver, typicallymaking 10 to 12 deliveries per day, must exert considerable physicalforce to first remove the ramp from its storage chamber, and then raisethe 60 to 80 pounds of front end dead weight of the ramp and pull itforewardly to attach it to the apron of the truck bed, each time adelivery is to be made. As a result of this, and particularly toward theend of a day when the truck operator may be tired and rushed, manydrivers have simply pulled the truck ramp from the stowing chamber andsought to operate it without attaching its front end to the apron of thetruck body floor. While operation in this manner avoids the physicalexertion required to raise the front end of the ramp and simultaneouslypull it forewardly, while its rearmost end is in engagement with, forexample, a very rough concrete surface creating considerable friction,the practice is regarded as unsafe and capable of causing injury to theoperator of the hand truck which is used to load and unload the vehicle,as well as to the cargo being transferred. The reason for this is thatthere will be a considerable drop between the floor of the cargo truckand the upper end of a ramp which has not been raised to the level ofthe truck bed and attached thereto. When cargo is being unloaded fromsuch a truck bed by hand truck and the wheels of the hand truck fallfrom the bed to the upper end of the ramp, injuries to the back of thehand truck operator who normally is the driver of the truck, can occurand, of course, cargo being carried on the hand truck may fall off thehand truck and drop a considerable distance to the ground. The practiceis also very hard on hand trucks, and can materially shorten theiruseful lives.

SUMMARY OF THE INVENTION

The improvements which will be first generally, and then particularly,discussed herein seek to significantly ease the physical exertionrequired to move the ramp from a stowed position to an operativeattached position, and to unhook it and restow it. Raise-assisting,weight counterbalancing mechanisms have been incorporated in an improvedramp structure to considerably decrease the lifting force which must beexerted by the operator to raise the front end of the ramp structure toa position in which no vertical lifting force is required to maintainthe vertical position of the ramp, and attachment to the apron of thetruck bed is greatly facilitated. In addition, members have beenincorporated with the ramp assembly at both ends of the ramp forassisting it to slide both on the ground and freely within the storagechamber, and members have been incorporated for preventing the metalramp assembly from engaging the metal storage chamber.

One of the prime objects of the present invention is to provide a rampassembly of the character described which is far easier to deploy andstow than known existing ramp structures of a similar nature, and soencourages the operator to correctly deploy the ramp during the manytimes each day he is required to unload cargo at many different andoften widely spaced locations.

Still another object of the invention is to provide a ramp structure inwhich the same interchangeable parts can be used to support and deployramps of differing length and weight, with the parts providing avertical zone in which the upper ends of the ramps are counterbalancedfor attachment, even though the weights of the ramps may differ, and theheights to which the front ends of the ramps must be raised to reachattaching position also differ.

Another object of the invention is to provide improved ramp structuresof the character described which are rugged and will withstand continualand rough usage, and which require considerable less maintenance andreplacement than formerly.

Still another object of the invention is to provide a versatile rampstructure of highly reliable character which is economical tomanufacture and can be marketed at prices which are well within themeans of truck operators.

Finally, still another object of the invention is to provide a greatlyimproved, stowable, ramp structure which promotes both personal safetyand the safer transport of valuable cargo to and from the truck.

Other objects and advantages of the invention will be pointed outspecifically or will become apparent from the following description whenit is considered in conjunction with the appended claims and theaccompanying drawings.

DESCRIPTION OF THE DRAWINGS

In the drawings, FIG. 1 is a schematic side elevational view showing aramp in operatively deployed position, attached to the rear end of thetruck vehicle, the broken lines indicating a ramp storage chamber and astowed position of the ramp;

FIG. 2 is a similar, side elevational view on an enlarged scale, takenpartly in section, and with broken lines illustrating the position ofthe ramp when it is initially pulled from the storage chamber, beforeits front end is attached in operatively deployed position;

FIG. 3 is a still further enlarged, fragmentary side elevational view,taken partly in section, particularly showing the various positions inwhich the weight counterbalancing mechanism may be connected, dependenton the weight of the ramp in use;

FIG. 4 is an enlarged, fragmentary, sectional, side elevational view ofthe rear end of the ramp only taken on the line 4--4 of FIG. 7 toillustrate the manner in which an anti-friction foot pad is provided onthe rear end of the ramp;

FIG. 5 is a considerably enlarged transverse sectional view, taken onthe line 5--5 of FIG. 6, which more particularly illustrates the stowedposition of the parts, the gas cylinders being omitted to moreparticularly illustrate the attachment fittings which mount thecylinders in selected position;

FIG. 6 is a side elevational view of the storage chamber frame, withparts being broken away to show portions of the stowed ramp assemblytherein;

FIG. 7 is a top plan view of the storing chamber frame with portionsbeing broken away to show the ramp stored therein;

FIG. 8 is an enlarged, side elevational view illustrating the resilientplastic apring members mounted within the storage chamber near the frontend thereof;

FIG. 9 is an enlarged, schematic, side elevational view, similar to FIG.3, which illustrates the static balance positioning provided by thecounterbalancing gas cylinders;

FIG. 10 is an enlarged, fragmentary, top plan view illustrating theconnection of the ramp arm assembly to the slide blocks; and

FIG. 11 is an enlarged, transverse sectional view of one of the memberswhich connect the gas cylinders to the ramp arm assembly.

Referring now more particularly to the accompanying drawings, and in thefirst instance particularly to FIG. 1, a typical four-wheeledcargo-carrying truck, generally designated T, is disclosed as having acargo container or compartment 10 supported on wheels 11 for movementalong a highway or road system to a loading or unloading location. Themotorized truck T in the usual manner includes an operator's driving caband engine (neither of which are shown) connected to the truck body. Thetruck body frame F typically includes transversely spaced longitudinallyextending chassis rails 12 beneath the compartment 10, on which bodylong sills 12a are fixed. The ramp storage chamber frame SF may besupported between these members on truck body frame cross members 13 bywelding hanger angles 14 to the body frame cross members 13, as FIG. 5particularly indicates. The storage channel frame SF typically includestransversely spaced, oppositely disposed, steel side channels 15 whichmay be connected at their upper ends by bridging cross members 16. Thelower flanges 15a of channels 15 comprise a storage channel floor ortrack, and the upper flanges 15b form the top of the storage chamberwhich is generally designated SC. The channel members 15 are also formedof steel, so as to be weldable to the frame members 13 and 14, andprovided at the rear ends of channel rails 15, in a fixed position onthe flanges 15a, are rugged synthetic plastic spacer plates 17 which areformed of a low friction, ultra high molecular weight plastic material,such as polyethylene. The material has a molecular weight in theneighborhood of 3.1 million.

As FIGS. 5 and 6 both indicate, a ramp assembly or ramp, generallydesignated R, is slideable into the open rear end of the storage chamberSC to move to a completely stowed position within storage chamber SC.The ramp assembly R, which is preferably formed from a lightweightmaterial such as aluminum, includes a floor 18 spanning side rails 19,and having traction projections 18a at longitudinal intervals integrallyformed in its upper surface. Inversely T-shaped, downwardly projecting,integral ribs 20 span the floor 18. At its rear end, the floor plate 18has a dependent stepped portion 18b (FIG. 4) which spans the side rails19 and slots (not shown) may be provided in the floor plate 18 toprovide hand grips. Alternatively, handles may be provided on the endsof the side rails 19 which can, of course, also be easily grasped by theoperator. At the front end of the ramp R, see FIG. 2, transverselyspaced downwardly dependent hooks 27 provided on the floor 18 areadapted to hook into the usual openings 28 provided in the rear apron 29of the truck body floor plate or bed 30.

Provided to connect the ramp R to the storage chamber SC, is a carriagearm frame generally designated A comprising a pair of arms 31 which eachpivotally connect at their rear ends to the ramp R. Transversely spacedfittings 32 (FIG. 3), welded to the floor 18 of the ramp R and itsdependent rib members 20, support pivot pins 33 on which the rear endsof arms 31 are pivotally received. At their front ends, arms 31 connectto a spanning rail 31a which has bearing block, anti-friction slideguides 34 pivotally received thereon. As FIG. 9 indicates, stops 35affixed to the rail 31a engage stops 36 which depend from the channelflanges 15b to prevent the carriage arm assemblies A from being drawncompletely out of the storage chamber SC. In this sense, they couple thecarriage arm assemblies A to the storage chamber frame members 15. Theguides 34, which ride along the spacers 17 at the front end of thestowing chamber SC, and then along the flanges 15a of the channel rails15, remain in position on these surfaces due to the weight of the ramp,and it is the arms 31 which pivot with respect to the blocks 34, whichmay be molded of the same ultra high molecular weight polyethylenematerial. Carriage arm assemblies A have been previously used withstowable ramps, but have not in any way relieved the operator of thenecessity of physically raising the upper end of the ramp R from theposition shown in phantom lines in FIG. 2 to the raised attachedposition.

Provided to counterbalance the weight of the front end of the ramp R andretain it in a statically balanced position anywhere within the range ofpositions indicated by arc 37 (FIG. 9) are a pair of gas cylinders 38which connect in predesignated positions to the arms 31 and to the rampR. It is to be understood that gaseous pressure within each of thecylinders 38 exerts a constant pressure on their pistons and yieldablyresists any compression of the piston rods 38a into the cylindercasings. Within the arc 37, the weight of the front end of the ramp R isso counterbalanced by the pair of gas cylinders 38 that it remains in acondition of static balance, due to the static friction at all threesets of pivot points A, B and C. Pivot points A may be considered to bethe two pins 33, pivot points B may be considered to be the connectionsof the cylinders 38 to ball fittings 41 received in one of the sets ofopenings 1-5 provided in the blocks 42 affixed to ramp R (FIGS. 3 and5), and pivot points C may be considered to be the connections of thecylinder rods 38a to the ball fittings 39 received in the matchingthreaded openings 1-5 provided in each of the arms 31. Each of thefittings 41 and 39 have threaded ends which are removably, selectivelyreceived in the threaded openings 1-5 provided in blocks 42 and arms 31respectively.

As FIG. 9 indicates, for ramps having overall weights of 80, 98, 105,120 and 140 pounds, a pair of gas cylinders 38, which each exert aconstant pressure of 225 pounds, provide the so-called "dead band" arc37. As the front end of the ramp R is raised by the operator manually,the moment arm from pivots A to the selected openings in the set ofopenings 1-5 in the arms 31 increases. When the front end of the rampreaches the lower end 37a of the arc 37, the front end of the ramp Rwill remain in position, if the operator releases it. With thecounterbalancing forces exerted by gas cylinders 38, the front end ofthe ramp R may be moved to any position in the arc 37 which is oppositetruck floor with virtually no exertion of pressure by the operator.

In the first removed position, illustrated in broken lines in FIG. 2,the piston rods 38a within the gas cylinders 38 are in a fullyretracted, fully operatively compressed position, whereas, when the rampis in the deployed position, the piston rods 38a are in an expandedposition. In the counterbalanced zone arc 37, the hooks 27 remain in theopenings 28. At its lower end, each gas cylinder 38 has a socket fitting38b permitting it to pivotally receive the ball end 39a of the pivotfitting 39. At its upper end, each of the gas cylinders 38 has a socketfitting 38c permitting it to be attached via the ball end 41a of fitting41 to a like set of openings in the openings 1-5 provided in the blocks42 which are welded to the lower ends of ramp crossbars 20.

In FIG. 3, gas cylinders 38 are attached in the number 1 position andexert their pressure counterbalancing through a moment arm x. Had thecylinders 38 been attached in the number 5 position, as indicated by thebroken lines in FIG. 3, the force exerted would have been exertedthrough a greater moment arm y. The same gas cylinders 38 are, for thisreason, capable of counterbalancing ramps of the range of weightsindicated in FIG. 9 in the dead band 37 position. Not only do thecounterbalancing cylinders 38 provide the static balancing indicated byarc 37, they also considerably lessen the force required to initiallylift them to the dead band 37 a position from the initial position ofdeployment shown in broken lines in FIG. 2. For example, with a rampweighing 140 pounds and cylinders 38 attached in the number 5 positionson arms 31 and blocks 42, only a force of 42 pounds is initiallyrequired. In this attached position of the cylinders 38, a dead band arcof 131/2 inches is provided. With a ramp weighing 105 pounds and thecylinders 38 attached in the number 3 positions provided by the threadedopenings in arms 31 and blocks 42, initial lifting force through themoment arm need be only 34 pounds and a dead band arc which extends 14inches is provided. As the weight of the ramp decreases, the initiallifting force required decreases, and the dead band length tends toincrease. Thus, it will be seen that, even though the moment arm throughwhich the force is exerted in the first position of deployment shown inbroken lines in FIG. 2 is relatively small, still gas cylinders 38provide a lift assist function of importance to keep the initial liftingforce which must be exerted by the operator within low limits. Push-downforces to compress the cylinders 38 below dead band 37 are similarly, ofcourse, maintained within low limits.

To assist the operator in moving the ramp R rearwardly to attach thehooks 27 to the apron 29, an ultra high molecular weight polyethylenefoot 43 of the same material is provided with a recessed shelf 44 whichreceives the dependent protrusion 18b on the lower end of the ramp. Themembers 43 include resilient curvilinear arms 45 and 46 which aredeformable to a spread position such that they will pass the bar 20aprovided on the lower end of the lowermost bar 20 and then resilientlysnap into a position in which the arms 45 and 46 abut the web 20a of therib member 20, while engaging the lower surface of the ramp floor 18. Atits upper end, the shoulder 44a of the shelf 44 will bear on the footpart 18b in this position, when the flange 20a is in engagement with thebody of the member 43. The foot 43 depends, to support the ramp sidewalls 20 at a predetermined distance 47 above the cement floor or thelike on which the very low friction foot 43 supports the ramp at groundlevel G.

When the ramp is to be stowed in the storage position illustrated inFIG. 6, the lower edges 20a of side walls 20 are supported at apredetermined distance above the metal floor formed by channel webs 15by the very low friction rear end spacers 17. At its front end, the rampstructure is supported by the slide blocks 34 to hold the entire rampstructure away from the lower flanges 15a of the storage chamber. Inthis position of the parts, the carriage arms 31 are folded to abuttingengagement with the ramp floor 18 and are positioned above the lowerflanges 15a of channels 15.

As shown in FIG. 8 particularly, ultra high molecular weight springs ofthe same polyethylene material, generally designated 48, are provided inthe position shown in FIG. 6 on the upper flanges 15b of the channels15. Each spring includes a portion 48a which attaches to a flange 15b bybolts 49, and has a downwardly dependent curvilinear section 48b, with abead type free bearing end 48c. Provided on the upper surface of theportions 48b are a series of collapsible resilient columns or pins 48dwhich can flatten out, when the ramp structure R deforms the springsupwardly and brings the pins 48d into engagement with the upper webs15b. When this occurs, the pins 48d are collapsed and the wholestructure of the springs 48 will cushion the ramp in the stowed positionand prevent its contact with flanges 15b. While curvilinear springs ofsteel have previously been provided, the low friction springs 48, withthe collapsible pins 48d, accomplish a greater cushioning whileproviding far less resistance to sliding of the ramp structure into thestowed position.

Provided in the present structure to prevent contact of the rampstructure with the web side walls of channels 15 are longitudinallyspaced guide blocks 51 which are bolted as at 51a to the sides 19 of theramp. These blocks 51 protrude a distance laterally beyond the sides 19,as shown in FIG. 5, but not far enough to engage the web walls ofchannels 15. They are provided at longitudinally spaced intervals alongthe ramp structure and function to prevent the metal channel walls 20from laterally contacting the channels 15. They are constructed also ofthe same ultra high molecular weight polyethylene and have very lowfrictional resistance.

THE OPERATION

When the particular ramp structure is being mounted to the ramp body,the stowing chamber SC is first securely welded in position in a mannerpreviously described. The ramp structure R will use the same carriagearm structure A and slide glides 34. If the ramp R is, however, to havea certain predetermined length, and consequently a certain predeterminedweight, gas cylinders 38, which will be used for all weights to exertthe same constant pressure, will be mounted by one of the matched setsof openings 1, 2, 3, 4, or 5, in arms 31 and blocks 42, dependent on theweight of the ramp and the dead band arc 37 which is to be achieved. Thedead band arc 37 must extend sufficiently so that no substantial liftingforce need be applied to raise the front end of the ramp 20 at a timewhen the ramp R is being pulled rearwardly to attach its hooks 27 in thetruck apron openings 28. The set of openings 1-5 which is chosenpredetermines the moment arm about pivots 33 through which the gascylinders 38 operate in raising the front end of the ramp. Because ofthe placement of the pins 33 near the front end of the ramp, the centerof gravity of the ramp structure is rearwardly of the pins 33. Once theangle of repose between the rods 38a and the arms 31 in the initialposition of deployment is broken by the operator physically raising thefront end of the ramp, the moment arm through which the cylinders 38operate increases as the front end of the ramp raises until a fullcounterbalancing of the front end of the ramp occurs in the band 37.Once the front end of the ramp reaches the so-called dead band arc 37 inwhich it is virtually weightless, it is far easier for thedriver-operator to pull the ramp structure R rearwardly and deploy thehooks 27 in attached position.

When the truck bed 30 has been loaded or unloaded the release of thehooks 27 is further easily accomplished and, once this has occurred, thefront end of the ramp R can be pushed from the position shown in solidlines to the position shown in broken lines. In this position of theparts, the moment arm has been reduced to a negligible 11/2 inches andthe carriage arm assembly A remains in the collapsed position in whichit is shown, because the force exerted by the gas cylinders is notsufficient to in any way change the relationship.

The operator need merely then proceed to the rear of the ramp structureto grasp the hand holds and raise the ramp to a position in which it canenter the storage chamber SC. The glide blocks 34, which are at thistime resting on spacer plates 17, slide forwardly along the plate 17 andthen proceed to and along the lower webs 15a to front end of the storagechamber SC and the stowed position of the ramp illustrated in FIG. 6. Atthis time, the metal ramp structure is prevented by polyethylene parts17, 34, 48 and 51 from contacting the metal walls of the storagechamber. The shock absorbing springs 48 are deformed upwardly and pins48d, which essentially are miniature compression columns, collapse tofrictionally and resiliently resist upward movement and vibration of thestowed ramp structure.

Although preferred embodiments of the invention have been illustrated inthe accompanying drawings and described in the foregoing detaileddescription, it will be understood that the invention is not limited tothe embodiments disclosed, but is capable of numerous rearrangements,modifications and substitutions of parts and elements without departingfrom the spirit of the invention which is defined in the followingclaims.

What is claimed is:
 1. The method of assembling a retractable ramp ofthe type which is stowed within a rearwardly open storage chamber withina vehicle frame below a vehicle floor provided in a vehicle for carryingcargo, and which in an operative deployed position is extended from thevehicle frame, tilted downwardly, and selectively connected to thevehicle frame to serve as a loading or unloading platform for thevehicle frame, the ramp being one of a series of ramps of differinglength having a range of weights, each of the ramps having a rear endadapted to engage the ground surface on which the vehicle is deployedand a front end adapted to be releasably connected to the vehicleadjacent the rear end of the vehicle floor, the retractable rampincluding an arm means pivotally connected to the ramp floor assemblynear the front end thereof and extending forwardly into the storagechamber to couple thereto, each of the ramps and the arm means having aplurality of attachment positions provided thereon, the steps of:a.selecting one of said series of ramps; b. pivotally connecting, betweenthe ramp and arm means, force exerting means to one of said attachmentpositions such that it will exert a force, through a moment arm ofincreasing length as the front end of the ramp is manually raised aboutthe pivotal connection of the arm means to the ramp as a fulcrum, whichmaintains the front end of the ramp in a position substantially oppositethe vehicle floor without the required imposition of upward or downwardmanual force; and c. removing the upward pressure exerted manually onthe front end of the ramp floor assembly, pulling it manually forewardlyto a location of attachment to the rear end of the vehicle adjacent thevehicle floor, and attaching it to the rear end of the vehicle adjacentthe vehicle floor.
 2. A method of operating a retractable ramp of thetype which, in inoperative position, is telescoped within a rearwardlyopen longitudinally extending storage chamber within a cargo carryingvehicle frame below a vehicle floor provided therein, and in operativedeployed position is extended from the vehicle frame, tilted downwardly,and releasably connected to serve as an unloading or loading platformfor said vehicle floor, the ramp having: an elongate ramp floor assemblyhaving a rear end adapted to engage the ground surface on which thevehicle is deployed and a front end adapted to be releasably connectedto the vehicle adjacent the rear end of the vehicle floor, an arm meanspivotally connected to the ramp floor assembly near the front endthereof and extending forwardly therefrom into the storage chamber tosupport the front end of the ramp floor assembly when the latter isinitially withdrawn from the chamber, force asserting, weightcounterbalancing means pivotally connected between the arm means andramp floor assembly and positioned to exert power to assist manualmovement of the ramp floor assembly from an initial position of extendedinoperative deployment when the force asserting means is more nearlycoextensive with the ramp floor assembly to an operative second positionof tilted deployment in which the front end of the ramp floor assemblyis raised relative to the arm means and can be connected to the rear endof the vehicle to serve as a platform leading to and from the vehiclefloor, the force asserting means being positioned so as to exert a forcethrough a moment arm of increasing length as the front end of the rampfloor assembly is manually raised about the pivotal connection of thearm means to the ramp floor assembly as a fulcrum such that, near theupper end of the raising movement, substantially no vertical manualforce is required to be exerted to maintain the front end of the rampfloor assembly in the vertical position in which it is deployed, tofacilitate the ready connection of the front end of the ramp floorassembly to the said vehicle adjacent to said vehicle floor, the stepsof:a. exerting upward pressure to raise said front end to a position inwhich the tilted ramp remains in raised position, b. removing the upwardpressure exerted manually on the front end of the ramp floor assemblyand pulling it manually forewardly to a location of attachment to therear end of the vehicle adjacent the vehicle floor.
 3. The method ofclaim 2 wherein said force asserting means comprise a pair of laterallyspaced constant pressure exerting cylinder and piston assemblies inwhich, acting through a predetermined moment arm, they together balancethe weight of the front end of the ramp floor assembly to retain it inposition at substantially the level of the vehicle floor without theimposition of manual upward or downward force.
 4. In a retractable rampof the type which in an inoperative position is telescoped within arearwardly open, longitudinally extending storage chamber within avehicle frame below a vehicle cargo supporting floor provided therein,and in operative position is extended from the vehicle frame, tilteddownwardly, and releasably connected to serve as an unloading or loadingplatform for said vehicle floor, the combination of:a. an elongate rampfloor assembly having a rear end adapted to engage the ground surface onwhich the vehicle is deployed and a front end adapted to be releasablyconnected to the vehicle adjacent the rear end of the vehicle floor; b.a floor defining said storage chamber; c. an arm means pivotallyconnected to said ramp floor assembly near the front end thereof andextending forwardly therefrom into the said storage chamber; d. atransport device provided on the front end of said arm means to supportthe front end of said ramp floor assembly when the latter is withdrawnfrom said chamber, movable along said chamber when the ramp floorassembly and arm means are to be stored in said chamber, said transportdevice comprising a pair of laterally spaced, low friction, syntheticplastic slide blocks to which said arm means pivotally connect, mountingfor sliding movement along said storage chamber; e. a pair of fixedsynthetic plastic slide plates at the rear end of the chamber; and f.said slide blocks supporting the ramp floor assembly and arm means abovethe floor of the storage chamber at the front end of the chamber whenthe ramp floor assembly and arm means are moving forwardly to stowedposition, while the fixed slide plates support the rear end of the rampfloor assembly in raised position above the floor of the storage chamberas the ramp floor assembly and arm means move to stowed position.
 5. Theinvention of claim 4 in which said storage chamber has side walls andlow friction, synthetic plastic plates protrude from said ramp floorassembly at longitudinally spaced intervals to prevent contact of saidramp floor assembly and storage chamber side walls.
 6. In a retractableramp of the type which in inoperative position is telescoped within arearwardly open, longitudinally extending storage chamber within avehicle frame below a vehicle cargo supporting floor provided therein,and in operative position is extended from the vehicle frame, tilteddownwardly, and releasably connected to serve as an unloading or loadingplatform for said vehicle floor; the combination with a vehicle havingsaid vehicle frame of:a. an elongate ramp floor assembly having a rearend adapted to engage the ground surface on which the vehicle isdeployed and a front end adapted to be releasably connected to saidvehicle adjacent the rear end of said floor; b. an arm means pivotallyconnected to said ramp floor assembly near the front end thereof andextending forwardly therefrom into the said storage chamber; c. atransport device provided on the front end of said arm means to supportthe front end of said ramp floor assembly when the latter is withdrawnfrom said chamber, and movable along said chamber when the ramp floorassembly and arm means are to be stored in said chamber; d. forceasserting, weight-counter balancing means pivotally connected betweensaid arm means and ramp floor assembly and positioned to exert power toassist manual upward movement of the front end of the ramp floorassembly from an initial position of extended inoperative deployment, inwhich the front end of the ramp floor assembly is removed from thestorage chamber and the transport device remains in the storage chamberto support the ramp floor assembly in a position in which the arm meansand force asserting means are more nearly coextensive with said rampfloor assembly to an operative second position of tilted deployment inwhich the front end of said ramp floor assembly is raised relative tosaid arm means and can be connected to the rear end of said vehicle toserve as a platform leading to and from said vehicle floor; e. saidforce asserting means being positioned so as to exert a counterbalancingforce through a moment arm of increasing length about the pivotalconnection of the arm means to the ramp floor assembly as a fulcrum asthe front end of the ramp floor assembly is manually raised such that,near the upper end of the raising movement, substantially no verticalmanual force is required to be exerted to maintain the ramp floorassembly in the vertical position in which it is deployed and readyhorizontal connection of the front end of said ramp floor assembly tosaid vehicle is facilitated.,
 7. The ramp of claim 6 wherein said forceasserting means is a pair of laterally spaced gas-filled cylinder andpiston assemblies with the gas therein maintained under a predeterminedconstant pressure.
 8. The ramp of claim 6 wherein a low friction,synthetic plastic, slide block is incorporated with the rear end of saidramp floor to depend therefrom and support the rear end of the rampfloor.
 9. The ramp of claim 8 wherein said ramp floor assemblyincorporates a cross bar of inverse T-shape at its rear end, and saidsynthetic plastic block has a shelf portion at its rear end forreceiving the rear end of said ramp floor and resilient arms forward ofsaid shelf portion grasping said cross bar from opposite sides thereof.10. The ramp of claim 6 wherein said ramp floor assembly has a group oflongitudinally spaced apart attachments near its front end for selectiveattachment thereto of one end of said force asserting means, and aseries of spaced apart attachments are provided for said arm means forselective attachment of the opposite end of said force asserting means,dependent on the weight of the end of the ramp floor assembly to bemanually raised.
 11. The ramp of claim 10 wherein, with each pair ofattachments in said series and group respectively, to which the ends ofsaid force asserting means may be connected, the increasing moment arm,as the front end of the ramp floor assembly is lifted, is such that theramp floor assembly will remain in the position in which it is deployedas the said front end approaches a level opposite the said floor of thevehicle.
 12. The ramp of claim 6 wherein said vehicle frame has openingsprovided therein adjacent floor level, and the front end of said rampfloor assembly has inverted hooks which are received therein.
 13. Theramp of claim 6 wherein said vehicle frame, near the front end of saidstorage chamber, has a synthetic plastic plate with a convex portiondepending into said storage chamber in the path of said ramp floorassembly to be compressed thereby, said storage chamber having a roofpart to which said plate is attached and said convex portion of theplastic plate having a series of upstanding, collapsible columnsthereon.
 14. The ramp of claim 6 wherein the front end of said arm meanshas a pair of laterally spaced, low friction, synthetic plastic slideblocks comprising said transport device, to which said arm meanspivotally connects such that said arm means does not contact the storagechamber, mounted for sliding movement in said storage chamber.
 15. Theramp of claim 14 in which the rear end of said storage chamber has apair of fixed synthetic plastic slide plates on which said slide blockstravel at the rear end of the storage chamber.
 16. The ramp of claim 6wherein said storage chamber has side walls and said ramp floor assemblyhas side rails, and exterior synthetic plastic blocks are attached tothe exterior side surfaces of said side rails to project laterally fromthe side rails at longitudinally spaced positions, said blocks beingslidably received in said storage chamber between the side walls thereofand preventing contact of the respective said side walls and side railsof the ramp floor assembly and storage chamber.
 17. The ramp of claim 6wherein said storage chamber has a floor and the rear end of the storagechamber has a synthetic plastic floor plate upstanding from the floor ofthe storage chamber, and said slide device has synthetic plastic blockportions positioned with respect to said arm means and ramp floorassembly such that said ramp floor assembly and arm means in stowedposition do not contact the floor of said storage chamber.